1. Field of the Invention
The invention relates generally to protective containers for storing and carrying magnetic cards, such as gift cards and credit cards and, more particularly to containers for magnetically shielding one or more cards stored therein, thereby preventing information on the card from being erased by strong magnetic and electromagnetic fields.
2. History of the Prior Art
Magnetic shielding prevents magnetic fields from reaching areas where they would otherwise cause magnetic interference or magnetic erasures. Magnetic shielding may be used around either the source of magnetic interference, to prevent electromagnetic radiation from leaving the source, or more typically, around a sensitive device, to prevent the electro magnetic interference from affecting operation of the sensitive device.
Permeability refers to a material's ability to attract and conduct magnetic lines of flux. The more conductive a material is to magnetic fields, the higher its permeability. Saturation is the limiting point of a material to conduct additional magnetic lines of flux. The saturation and permeability characteristics of a material are inversely related, therefore the higher a material's permeability, the lower its saturation point. Attenuation is a ratio used to measure the effectiveness of a given shield. Only magnetic materials are permeable. Non-magnetic materials—such as glass and wood—that allow magnetic lines of force to pass through them, are nonpermeable.
Unlike light in the visible spectrum, a magnetic field cannot be blocked or reflected; it can only be redirected. The use of shielding made of special shielding alloys possessing high permeability is the most effective way to redirect a magnetic field. These special alloys work by being attracted to the magnetic field, serving as a path for magnetic lines of flux so that they are diverted to the shielding material itself, thereby greatly reducing the strength of the magnetic field. It is important that the magnetic shielding offers a complete path for the magnetic field lines, so that they do not exit the material in a place where they will cause unintended interference or erasures. The most effective shielding alloys are about 80% nickel and 15% iron by weight, with the balance being copper, molybdenum or chromium, depending on the recipe being used. Mumetal®, CO-NETIC AA® and NETIC S3-6® are trademarks for alloys which have high magnetic permeability and provide magnetic field attenuation when used as magnetic shields.
Closed shapes are the most efficient for magnetic shielding. In buildings, the wall, floor and/or ceiling areas may have magnetic shielding panels applied to intercept the magnetic interference before it enters sensitive areas. These room-sized shields must have a relatively large area in order to prevent magnetic fields from coming around them. Unlike some waves, magnetic fields do not travel in straight lines, but are in loops, starting from the magnetic radiation source and eventually returning there. Although shield calculation formulas do exist, they are usually valid only for theoretical conditions of closed shield shapes and well-described interference fields.
Credit cards typically have a stripe which incorporates a thin layer of ferromagnetic particles. Information can be “written” on the card by selectively magnetizing regions of the ferromagnetic layer. Likewise, the card can be read by scanning the ferromagnetic layer and decoding the selectively magnetized regions. Unfortunately, when the entire card is subjected to a strong magnetic field, all information within the ferromagnetic layer will be erased.
Smart cards having an embedded integrated circuit are also becoming popular. A typical smart card incorporates a radio-frequency identification (RFID) tag. An RFID tag is usually a passive (having no on-board power source, such as a battery) and generally includes an antenna and an application specific integrated circuit (ASIC). The RFID tag receives its operational energy from a reader device, which must be in close proximity. Within what is termed the surveillance zone, the reader generates sufficient power to excite, or interrogate, the RFID tag. When radio frequency energy emanating from the reader antenna impinges on the tag, a current is induced in tag antenna. This induced current is routed to the ASIC, which then performs an initialization sequence. When the reader ceases transmitting its energy transmitting interrogation signal, the ASIC begins to broadcast its identity and any other requested information over the tag antenna. The tag transmission process utilizes low-energy transmission technology that selectively reflects the electromagnetic energy back to the reader at the same fundamental frequency as it was received, using the tag antenna as an energy radiator. The transmit/receive frequency employed is generally application dependent. Commonly available proximity interrogation systems operate at frequencies in a range of 60 kHz to 5.8 GHz, and typically employ frequency modulation for data transmission. Information reflected by the tag 102 is decoded by the reader 101. RFID tags can also incorporate memory (64 kilobytes of memory is now common), which can be of a read-only type or of a read and write type. In any case, the circuitry on an RFID tag is sensitive to both static electricity and strong magnetic fields. Static electricity can have voltage levels of thousands of volts—enough to fry the delicate components in an integrated circuit which typically have operating voltages of less than 5 volts. U.S. Pat. No. 6,553,481 describes a smart card having on-board memory. This patent is hereby incorporated by reference into this application.
What is needed is a protective case which protects credit cards and smart cards from strong stray magnetic fields and static electricity.